This project is concerned with a comparative analysis of ionic current channels in nerve and heart cell membranes and the relationship of these channels to excitability in both preparations. During the past year, the primary experimental preparations have been embryonic chick heart cells and giant squid axons. One major finding has been the observation of the pacemaker current component (I-f) in single chick heart cells with the perforated patch-clamp voltage-clamp technique. This technique allows us to record the If component without the loss of intracellular factors which have been shown to regulate this channel, and which may be lost using the more traditional suction pipette technique. Another major finding has been the resolution of a paradox concerning ion permeation of the delayed rectifier potassium ion channel, (I-K), in squid axons. The dependence of the (I-K) current-voltage relation on external potassium ion concentration is consistent-with independent movement of ions through the channel, which is at odds with tracer flux measurements from (I-K). The latter results are consistent with coupled movement of ions through the channel. We have developed a model which resolves this paradox.